The present invention relates to storage devices for computer systems. In particular, the present invention relates to optical and magneto-optical storage devices.
In optical and magneto-optical storage devices for computer systems, a beam of light is projected on to a disc surface that reflects the light in to a sensor. The surface of the dine is patterned to represent data that is typically stored in concentric tracks around the disc. The data is read from the disc by positioning the beam of light over a particular track on the disc and sensing the intensity and polarity of the reflected light from the disc.
To position the beam of light over a particular track, the art currently uses a course actuator in combination with a galvo-mirror assembly. The course actuator moves a lens assembly and a turning mirror over the disc. The light beam is projected toward the turning mirror, which reflects the light beam down into the lens assembly causing the light to focus on a track on the disc. The galvo-mirror assembly is used to direct the light toward the turning mirror. By applying an electrical current to the galvo-mirror assembly, the angle at which the light beam strikes the turning mirror and the lens assembly can be changed. By changing the angle at which the light beam is incident on the lens assembly, it is possible to move the light beam across one or more tracks while keeping the lens assembly fixed over a particular track. Thus, the galvo-mirror assembly provides fine control of the light beam.
Although the position of the light beam can be changed quickly using the galvo-mirror assembly, the light beam provided by the galvo-mirror system is less than ideal. In particular, at some angles of incidence of the light beam on the lens, the light beam can suffer from coma and astigmatism. Coma causes the spot of light on the disc to have a flare distribution and astigmatism causes different polarities of the light to focus at different distances relative to the surface of the disc. Thus, a fine control actuator is needed that can move the spot of light on the disc without causing as many imperfections in the spot of light.